For high frequency field, we set \(\omega =0.7m\) , \(\tau =45m^{-1}\)  for which the electron-positron pair is predominantly produced by multiphoton absorbtion.
For \(b=0\) , the external field has a constant frequency \(\omega _{\text{eff}}(t)=\omega \)  and we would recover the result in Fig. 1 of Ref. [1]} where the decreasing spatial scale affects the discrete \(n\) -photon absorption spectrum  [2]}, [1]}.
